Anencephaly is characterized by an open defect in the calvaria and skin, such that the cranial neural tube is exposed. It is a severe defect and is not compatible with survival. Infants that are alive at birth generally die within hours, but occasionally survive for a few days or weeks. The clinical features, diagnosis, and management of anencephaly are discussed in this topic review.

Anencephaly is one of the three major neural tube defects (NTDs). The others are encephalocele and myelomeningocele, which are discussed in separate topic reviews:

●Encephalocele is a herniation of the brain and/or meninges through a defect in the skull (cranium bifidum) that is "closed" or covered with skin. (See "Primary (congenital) encephalocele".)

The central nervous system (CNS) appears as a plate of thickened ectoderm called the neural plate at the beginning of the third week of embryonic life. The lateral edges of the neural plate become elevated to form the neural folds and fuse to form the neural tube; the fusion begins in the cervical region and proceeds in both the rostral and caudal directions (figure 1). The rostral neuropore closes on the 25th day after conception, and the caudal neuropore closes two days later [1]. Neural tube defects (NTDs) result from failure of the neural tube closure between 25 and 27 days after conception. The embryology of the neural tube is discussed in a separate topic review. (See "Closed spinal dysraphism: Pathogenesis and types", section on 'Normal cord development in humans'.)

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